Added methods for working with user/person affiliations

Added `.student?`, `.employee?`, etc. methods to user. These were also added to the person model. Added a method called `affiliations_to_sym` that returns an array of affiliations that are converted to symbols. Related to this issue https://github.com/biola/buweb-content-models/pull/15.
The attribute `graduation_major` doesn't exist on the Person model.